The vacuolar type H+ATPase (V1Vo- or V-ATPase) is a fundamental component of all eukaryotic cells. The complex is found in the membranes of a wide variety of intracellular compartments like clathrin-coated vesicles, chromaffin granules, endosomes, lysosomes, synaptic vesicles, Golgi derived vesicles and the yeast vacuole. In higher eukaryotes, V-type ATPases are also found in the plasma membrane of polarized cells such as osteoclasts and renal epithelial cells. Structurally similar ATPases have also been identified in the plasma membrane of Archaea and bacteria, where they are called A-ATPases and bacterial A/V-ATPases, respectively. The proton pumping action of the vacuolar ATPase plays a vital role in a large number of intra- and inter- cellular processes. In eukaryotic cells, these processes include receptor mediated endocytosis, protein trafficking, pH maintenance, storage of metabolites and neurotransmitter release. In polarized cells of higher eukaryotes, a vacuolar type ATPase is pumping protons across the plasma membrane leading to an extra- cellular acidification. Acidification of the enclosed space between the ruffled membrane of osteoclasts and the bone surface plays an important role in bone resorption and remodeling. Defects in the human vacuolar ATPase have been associated with a number of diseases such as renal tubular acidosis, sensorineural deafness, osteoporosis, diabetes and cancer. Fighting these diseases on a molecular level will require a detailed understanding of the structure and mechanism of the eukaryotic V-ATPase complex, which is the long term goal of this project. The Specific Aims of the now proposed work on the vacuolar ATPase are: (1) molecular structure and function of the vacuolar ATPase proton channel domain and (2) molecular structure and function of the V1 - Vo interface. In the first Aim, we plan to determine the atomic resolution x-ray crystal structure of the yeast vacuolar ATPase proton channel domain. In addition, we propose experiments to elucidate aspects of the mechanism of proton translocation across the isolated V-ATPase membrane domain. In the second Aim, we propose to determine the atomic resolution crystal structure of the subunit EGChead peripheral stalk complex and we will determine the molecular interactions that define the interface connecting V1-ATPase with the Vo proton channel domain. Results from the proposed studies will provide important molecular information on the mechanism of proton translocation and how the catalytic V1 ATPase sector and the membrane bound Vo proton channel domain interact to form a coupled enzyme complex. The proposed work will also shed light on the, as of yet poorly understood mechanism of V-ATPase activity regulation by regulated reversible enzyme dissociation and re-association, a mechanism now found to be involved in the development and maturation of cells in higher animals including human.